Feature request: /plugin update --all (bulk update all installed plugins)

### Describe the feature or problem you'd like to solve

Updating multiple Copilot CLI plugins requires running /plugin update individually. No bulk update exists.

### Proposed solution

/plugin update --all — updates all installed plugins, prints a summary of what changed.

### Example prompts or workflows

Standard QoL pattern (npm update, brew upgrade, gh extension upgrade --all). As the plugin ecosystem grows and the number of installed plugins grows, this becomes essential for easy update management.

### Additional context

_No response_

Start at the existing /plugin update command and trace how a named plugin is updated. Extend the flow to accept --all, process every installed plugin, and print a summary of what changed; done means bulk updates work without naming plugins and existing single-plugin behavior remains intact.

Written by the indexing model from the issue text.